An X-ray computed tomography (CT) apparatus is an apparatus which rotates an X-ray source and an X-ray detector around an object in a state of being disposed to oppose each other, applies X-rays from a plurality of rotation angle directions (views), detects X-rays (projection data) transmitted through the object for each view, and generates a tomographic image of the object on the basis of detected projection data. PTL 1 discloses a technique in which a collimator unit is dynamically controlled in the scanning starting vicinity and in the scanning finishing vicinity in a case where helical scanning is performed with a multi-slice CT apparatus (an X-ray CT apparatus provided with multi-column X-ray detectors), and thus only a region of an examination target object substantially used for image reconstruction is irradiated with X-rays so that an exposure dose is reduced.